This present invention generally relates to a propeller blade constructed from a plastic material such as a fiber-reinforced plastic material, and more particularly to an arrangement for protecting the structural integrity of a plastic propeller blade as well as indicating the occurrence of foreign object damage (FOD) to a leading edge of the propeller.
Fan blades of turbo engines and propellers have previously been manufactured from composite materials including fiber-reinforced plastic materials. These materials give great strength and low weight, but are detrimentally effected by a high sensitivity to rain and particle erosion (e.g. insects). Further, such composite fan blades and propellers are easily damaged by impinging large-size foreign bodies, such as chunks of ice, rocks, birds, etc. (FOD).
The above-noted drawbacks apply particularly to the slim, sharp leading edges of such composite-material propellers or fan blades, all the more so with increasing relative velocity between the propeller and the impinging particle. At the same time, the need to maintain high aerodynamic efficiency does not permit any changes in profile of the leading edge.
A known practice to protect, e.g. wooden propellers of composite-material helicopter rotor blades, has been to provide them with a metal coating or cladding on the leading edge. However, while FOD to translucent and impact-resistant fiber materials, such as glass fiber reinforced plastic materials and aramide fiber-reinforced plastic materials, is indicated by the resultant surface delamination, even the most severe internal FOD often remains invisible in carbon fiber-reinforced materials and therefore are quite difficult to detect. However, in the known practice of protecting propeller blades with a metal coating or cladding, no provision is made for indicating severe FOD to the structure of the blade, i.e. give early warning without sacrificing the protective functions.
Icing risk is another problem afflicting propellers. Ice buildup on the blades greatly impairs aerodynamic propulsion, and the high-velocity ice particles hurled off the propeller additionally jeopardize the airframe structure. Also, considerable engine imbalances may result.
Several known deicing methods exist, such as by pulsating inflatable rubber air cells (Goodrich anti-ice boot), by chemical deicing through alcohol or glycol spray (see FAA Advisory Circular 20-117), by hot bleed air from the engine , or by electrical resistance heating conductors fitted to the airfoil.
Another problem with plastic propeller blades is posed by the fact that such propellers or profan rotors are among the structures most probably struck by lightning. Carbon fiber-reinforced plastic materials are especially sensitive to lightning damage owing to their partial electrical conductivity in that the fiber is conductive while the plastic matrix is insulating.
Therefore, it is an object of the present invention to provide a propeller blade constructed from a composite material with protection from erosion, FOD and icing in combination with lightning protection in a complemental, redundant arrangement.
It is a further object of the present invention to provide a propeller blade constructed from a composite material with a metal shroud at the leading edge of the propeller which is spaced from the plastic airfoil of the blade by a cavity having electrical conductors.
The combination of features provided in accordance with certain preferred embodiments of the present invention provides a number of advantages. For example, the metal shroud spaced apart from the plastic airfoil of the propeller blade proper protects the airfoil from the erosive effects of small particles or rain drops. When a large-size body impinges, as perhaps a bird, the metal shroud is dented to absorb sufficient impact energy to prevent damage to the plastic profile behind it. The deformation is recognized also at surface inspections, enabling the damaged blade or metal shroud to be replaced. Deformation of the metal shroud prevents the blade from being weakened structurally; it merely impairs the aerodynamic flow around the blade at this point. Severe deformation of the metal shroud suggests possible internal FOD to the plastic propeller and therefore further inspection of the blade.
The cavity provided between the metal shroud and the plastic airfoil can advantageously be utilized for ducting hot deicing air, which is diverted from the compressor or consists of fresh air routed through an exhaust gas heat exchanger, so that icing of the critical airfoil leading edge is prevented. Provision for blowing off the air from the propeller blade can be made at the joints of the metal shroud or at the blade tip.
Also providing deicing features is the advantageous provision of an electrical heating conductor connected to the metal shroud to conduct heat. This serves to effectively heat the metal shroud at a stagnation point area, which is especially jeopardized by icing, while simultaneously protecting the temperature-sensitive plastic material, the air space between the metal shroud or heating conductor and the plastic airfoil serving to provide thermal insulation.
Additionally, the metal shroud advantageously discharges current when lightning hits the plastic propeller blade, so that the flash will not stress current-sensitive composite fiber materials. Serving to discharge the lightning current is also the grounded ground return of the electrical heating conductor.
In a further advantageous feature of preferred embodiments of the present invention, a second metallic inner shroud is conformally fitted to the plastic propeller blade surface and is connected to the outer metal shroud. This inner shroud is preferably arranged near a hub area of the propeller blade and gives additional protection of the plastic airfoil in the event of FOD and permits improved fixation of the outer metal shroud. Preferably, the inner shroud is welded to the outer metal shroud and bonded to the plastic airfoil. The weld joint is preferably roller seam welded or alternatively spot welded.
In an advantageous embodiment of the present invention, the metal shroud has a number of sections arranged along the direction of blade span. This helps to offset thermal expansion of the metal shroud and reduces the risk of fatigue fracture as a result of different moduli of elasticity of metal and plastic, respectively. This also enables the replacement of individual sections upon damage.
Preferred materials for the metal shroud are titanium, titanium alloys, INVAR steel and maraging steel alloys.
In a preferred embodiment of the spanwise edge of the metal shroud and the plastic airfoil, the adhesion of the edges under centrifugal load is improved by giving them a zig-zag or circular arc shape.
In an advantageous feature of preferred embodiments of the present invention, the heating conductor takes the shape of a coaxial conductor having an inner conductor, an insulating layer enveloping it, and a metallic shroud tube. This permits the heating conductor to be advantageously connected to the metal shroud by brazing or other process. The shroud tube also serves to increase the metallic cross-sectional area and so reduces the conductor resistance to improve the current discharge when lightning strikes.
In a preferred embodiment of the present invention, the heating conductor is brazed to the metal shroud in sections, and at intermediate sections is curved inwards to form expansion bends. This serves to reduce mechanical and thermal stresses caused in the heating conductor by differences in temperature and thermal expansion between metal and plastic. In one embodiment of the present invention the heating conductor is attached by regularly spaced heat conduction lugs, which improves the dissipation of heat to the metal shroud and advantageously stiffens the shroud.
Deicing/heating of the radially outer propeller areas is not invariably required, because the high peripheral speed prevents icing. In a further feature of preferred embodiments of the present invention, therefore, the heating conductor is provided only along a portion of blade span and is eliminated wherever continuous heating is difficult for lack of space in the sharp-edged outer zone of the propeller airfoil or where the heating conductor is not durable enough to safely withstand the increased flexural vibration in the upper blade section. In an advantageous embodiment, the heating conductor is then looped with forward and return lines running parallel to produce uniform heating of the metal shroud.
In a further feature of the present invention, the radially outer tip of the plastic propeller blade is clad with a metallic protective cap which electrically conductively connects to the metal shroud and the shroud tube of the heating conductor. This cap forms a flash entry or exit point, which is especially threatened by destruction through lightning, and it additionally protects the blade tip of the plastic propeller from mechanical damage. The conductor area of the lightning protection provision is preferably increased by installing one or several metallic conductor strips to run the length of the blade from the protective cap to the propeller hub and grounding them. In a preferred embodiment, electrically insulated flat metal conductors are formed, bonded and embedded into the pressure side of the airfoil below an erosion protection skin extending over the entire propeller blade.
Given suitable design and selection of material these lightning conductors are a little less durable than the plastic propeller blade. This provides a substantial advantage, considering that fatigue failure of the lightning conductors can be detected by electrical resistance test between the blade tip and the hub to give a simple maintenance criterion for further airworthiness checks on the propeller system.
The advantages provided by the propeller blade protection system of preferred embodiments of the present invention, therefore, include its deicing capability and simultaneously, its redundancy with respect to multiple lightning strikes, its suitability for testing by simple visual inspection and simple electrical resistance check, and the ease with which damaged individual components can be replaced.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.